In vitro experimental studies, undertaken by us and others, dealing with the effects of injury/stabilization have revealed that the kinetics across the injured/stabilized and adjacent joints of the lumbar spine change following surgery. However, it is neither practical to test a specimen subjected to complex loads seen in vivo nor possible to estimate experimentally the stresses and strains within the spinal structures. using this approach. It is also not practical to simulate the effects of muscles, lordosis, etc. The quantification of the loads imposed on spinal elements of normal and surgically altered spines is also difficult. Likewise, it is almost impossible to address all the parameters which can be varied within a given fixation system as well as the large number of fixation systems available. The clinical issue of screw (implant) loosening/breakage still needs to be addressed. All this mandates development of analytical models to complement the experimental studies. This proposal seeks support to develop three-dimensional non-linear finite element models of intact lumbar spine motion segments, one (L4-5), two (L4-Sl), and three (L3-Sl), using the CT technique. The models wilt be modified to simulate the effects of lordosis, and a number of clinically relevant injuries and fixation devices spanning one and two motion segments. The model responses to various load types will be computed using a commercially available finite element package - ANSYS with the Alliant Super mini-computer. Appropriate experiments will be undertaken to complement the finite element models. It will be possible, thereafter, to compare stress data of the injured/stabilized models with the corresponding intact models to identify regions of high and low (abnormal) stresses. Regions of abnormal stresses may initiate bone or implant (screw) failure. Regions of stress-shielding may lead to bone resorption and implant loosening. Both phenomena, screw loosening and failure, are clinically significant. Comparisons will also be made for the changes in intradiscal pressure, stresses, and other relevant parameters as a result of injury/stabilization at both the affected and adjacent levels. The effects of one level fixation vs. two level fixation will be analyzed. It is hoped that these analyses will further our basic understanding of the effects of injury and stabilization procedures on the human spine. Such analyses may afford us an opportunity to identify parameters that will help design an optimal fixation device.